Heretofore, several proposals have been made with reference to dry planographic plates of the type having an ink-repellent silicone rubber layer and a photo-sensitive layer comprising essentially an ethylenically unsaturated monomer and a photoinitiator. When these plates are exposed to light, the photo-sensitive layer polymerizes and/or crosslinks to become insoluble or to adhere firmly to the adjacent silicone rubber layer. In the case of the silicone rubber layer over the photo-sensitive layer, the silicone rubber layer in unexposed areas is washed out together by dissolving the underlying photo-sensitive layer in unexposed areas as described in U.S. Pat. No. 3,677,178. In another case, as described in U.S. Pat. No. 3,894,873, the photo-sensitive layer adheres firmly to the overlying silicone rubber layer upon exposure to light, and the silicone rubber layer is rubbed off only in unexposed areas without dissolving the photo-sensitive layer.
Furthermore, in the case wherein the silicone rubber layer is positioned beneath the photo-sensitive layer as described in U.S. Pat. No. 4,086,093, only unexposed areas of the photo-sensitive layer are dissolved to make non-image areas.
In these dry planographic plates, there exists a boundary layer in which the silicone rubber layer and the photo-sensitive layer are mixed, because the photo-sensitive layer is directly adjacent to the silicone rubber layer or to a thin intermediate layer intervening between two layers. Long time storage duration results in further curing of the silicone rubber in this boundary layer, and then the bonding strength between the silicone rubber layer and the photo-sensitive layer increases and it also becomes difficult to dissolve the photo-sensitive layer. Therefore, in the case wherein the silicone rubber layer is positioned over the photo-sensitive layer, it becomes difficult to wash out the silicone rubber layer in unexposed areas by dissolving the photo-sensitive layer, or to rub it off selectively. In the case wherein the silicone rubber layer is positioned beneath the photo-sensitive layer, unexposed areas of the photo-sensitive layer become difficult to dissolve.
For the above reasons, the dry planographic plates of the prior art have been subjected to the disadvantage of relatively short shelf life. Furthermore, the photo-sensitive layer comprising essentially an ethylenically unsaturated monomer and a photoinitiator tends to polymerize rapidly and is thermally unstable even in the dark. Thus, various inhibitors have usually been added to the photo-sensitive layer. Since these inhibitors are effective in inhibiting thermal polymerization but also tend to restrain photo-polymerization, enough of these inhibitors may not be added.
An object of the present invention is to overcome such deficiencies and to provide a dry planographic plate having the advantages of planographic plates of the prior art and having a longer shelf life as well.